Modified adsorbent for capturing heavy metals in aqueous solution

ABSTRACT

A method and adsorbent composition for removing heavy metals from contaminated water, comprising passing a stream of water having a concentration of one or more heavy metals through an adsorbent comprising granules of a mixture of from about 1 wt % to about 15 wt % clay and a thermoplastic polymer matrix; and collecting water having a reduced concentration of said heavy metal(s) downstream of said adsorbent.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of copending U.S.application Ser. No. 12/759,081, filed on Apr. 13, 2010, the contents ofwhich are hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to an adsorbent for removing heavy metals fromcontaminated water, and more particularly, to a continuous process fordecontaminating a heavy metal contaminated stream of water.

BACKGROUND OF THE INVENTION

In the area of water treatment, such as ground water or industrial wastewater treatment, there is an ever-increasing need to remove undesirableand even toxic contaminants, particularly heavy metal contaminants, fromwater. Many industrial processes utilize aqueous solutions of heavymetals, such as lead in manufacture of batteries, and chromium or copperin electroplating solutions.

Unfortunately, the removal of such heavy metals from the aqueoussolutions used in these processes has proven to be not only difficultbut expensive, especially when conducted in batch processing. In orderto reduce the cost and time of decontamination, prior art continuousprocesses have utilized quite expensive synthetic ion-exchange resinspacked in a column, through which the contaminated water is passed fortreatment and heavy metal removal. Typically, such syntheticion-exchange resins require reverse flushing of the packed column toregenerate the resin, whereby a heavy metal-contaminated flushing fluidis produced, making disposal still difficult.

U.S. Pat. No. 4,133,755 to Tarao et al., incorporated by referenceherein, discloses agents for removing heavy metals comprising acomposition consisting mainly of a dithiocarbamate bond-containing lowmolecular weight compound, amorphous silica and activated carbon powder,granulated with a vinyl acetate polymer binder and clay as a thixotropicexcipient. The patentees disclose packing said granulated materials intocolumns for treating mercury-contaminated waste water.

U.S. Pat. No. 5,667,694 to Cody et al., incorporated by referenceherein, discloses a process for removing dissolved heavy metalsincluding lead and radioactive contaminants from contaminated aqueoussystems including aqueous soil systems. An organically modified smectiteclay, or organoclay, is used to treat these systems. Organoclays are thereaction product of smectite clays and quaternary ammonium compounds.The organoclay is brought in contact with system to be treated where itsorbs the heavy metal in the aqueous system onto the organoclay whichsorbed complex is then removed by a variety of methods includingflotation and air sparging.

U.S. Pat. No. 5,256,615 to Oomura et al., incorporated by referenceherein, discloses a granular inorganic ion exchanger which is obtainedby firing at 400° C. or higher a granular molded product of a mixture ofa metal alkoxide such as Si(OMe)₄ or hydrolyzate thereof, a clay mineralsuch as sepiolite and an inorganic ion exchanger such as antimonypentoxide and which has mechanical strength and heat resistance withoutlosing its inherent ion exchangeability.

U.S. Pat. No. 5,045,210 to Chen et al., incorporated by referenceherein, discloses an ion-exchange media comprising a modifiedpolysaccharide material and a modified particulate polymeric material,said modified materials comprising a polysaccharide and particulatepolymeric materials covalently bonded to a synthetic polymer, saidsynthetic polymer comprising a copolymer made from a polymerization of:(a) a polymerizable compound having a chemical group capable ofcovalently coupling, directly or indirectly, to said materials; and (b)a polymerizable compound containing (i) an ionizable chemical group or(ii) a chemical group capable of transformation to an ionizable chemicalgroup. The media is useful to selectively remove heavy metalcontaminants from aqueous solutions containing said contaminants.

World Patent Publication No. WO 00/72958 to Payzant et al., incorporatedby reference herein, discloses a networked polymer/clay alloy producedfrom a monomer/clay mixture comprising a monomer, a cross-linking agentand clay particles. The clay is chemically integrated with the polymersuch that, on exposure to water, the networked polymer/clay alloy swellswith substantially no clay separating from the alloy.

However, none of the above-discussed references discloses or suggests arelatively inexpensive but highly effective ion-exchange packing forremoval of heavy metal contaminants from contaminated water streams.Accordingly, there exists a need in the art to overcome the deficienciesand limitations described hereinabove.

SUMMARY OF THE INVENTION

In a first embodiment, the invention is directed to an adsorbentcomposition for metal ions, comprising a granulated mixture of fromabout 1 wt % to about 15 wt % clay and a thermoplastic polymer matrix.

In another embodiment, the invention is directed to a method forremoving heavy metals from contaminated water, comprising passing astream of water having a concentration of one or more heavy metalsthrough an adsorbent composition comprising granules of a mixture offrom about 1 wt % to about 15 wt % clay and a thermoplastic polymermatrix; and collecting water having a reduced concentration of saidheavy metal(s) downstream of said adsorbent.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in the detailed description whichfollows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the present invention.

FIGURE shows a recirculating system for treating heavy metalcontaminated water according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to an adsorbent composition for removing heavymetals from contaminated water, and more particularly, to a continuousprocess for decontaminating a heavy metal contaminated stream of water.

Naturally occurring clay has been known for use in treating watercontaminated with heavy metals, since some such clays have an inherention-exchange capacity. For example, clay minerals have been found to bein the form of layered molecular structures, often having variableamounts of iron, magnesium, alkali metals, alkaline earths and othersuch cations sandwiched between the layers, which cations can exchangewith heavy metal cations in aqueous solution.

In “Saudi Arabian clays for lead removal in wastewater”, Applied ClayScience, 42:671-674 (2009), by S. A. Al-Jlil and F. D. Alsewailem,incorporated by reference herein in its entirety, several naturallyoccurring Saudi Arabian clay materials were tested for lead adsorptionefficiency from wastewater in batch processes. In some experiments,pretreatment of the clay with hydrochloric acid was found to increaselead adsorption efficiency, while other clays performed better in theabsence of pretreatment. Additionally, adsorption efficiency wasdemonstrated to increase with increasing pH of the lead-contaminatedwater solution.

However, clays cannot be used in a continuous process due to theirbulkiness and relatively low structural strength. As such, clay hastypically been used in only batch processing, since packing anion-exchange column with naturally occurring clay for continuousprocessing results in tightly-packed clay, especially when the processis conducted under pressure. Such tight packing ultimately reduces orcompletely blocks the process flow. Additionally, clay packings have atendency to channel under high pressure, resulting in reduced contactbetween the heavy metal contaminant ions and the naturally occurringcations in the clay matrix, reducing overall efficiency in heavy metalremoval from the process stream.

In an effort to address these deficiencies, the present inventors havediscovered a new adsorbent composition for heavy metal ions which aredissolved in aqueous media, which packs in a column more uniformly andwith better porosity than clay particles alone, and is much moretolerant of applied pressure.

In one embodiment, an adsorbent composition for metal ions is disclosedcomprising a granulated mixture of from about 1 wt % to about 15 wt %clay, even between about 3 wt % and 10 wt %, or even between about 4 wt% and 7 wt %, such as about 5 wt % clay, and a thermoplastic polymermatrix, and preferably having less than about 1 wt % of additionaladsorbents. Advantageously, the thermoplastic polymer is one having amelting or softening point above about 180° C., which permitsmelt-mixing of particles of clay with the polymer to form the claimedmixtures.

In a preferred embodiment, the thermoplastic polymer is polyethyleneterephthalate (PET) or polystyrene (PS), both of which polymers have asuitable affinity for clay particles and are readily melt processedaccording to conventional techniques. The polymers can be so-called“virgin” polymers, i.e. newly formed polymers, or can be derived frompolymer scrap, such as recycled polymer, which is a particularlyadvantageous embodiment of the invention, as it permits reuse of polymermaterials which might otherwise be discarded, necessitating disposal ina landfill or the like.

The adsorbent clay is preferably one which has a naturally-occurringion-exchange capacity, such as an Illite clay. Illite clay is anon-expanding, micaceous mineral, classified as a phyllosilicate orlayered aluminosilicate, which has a repeating layered structure oftetrahedron-octahedron-tetrahedron molecules, wherein the interlayerspaces are occupied by poorly hydrated potassium cations. Oneparticularly suitable such clay is found in the north region of SaudiArabia. The present inventors have found that, likely because of theexcellent dispersion of clay particles in the adsorbent composition, incombination with the excellent structural strength of the overallcomposition, it is unnecessary to incorporate additional known adsorbentmaterials, such as activated carbon, silica, or the like, to thepresently disclosed adsorbent to achieve good heavy metal removal fromindustrial waste streams. As such, the presently disclosed adsorbentpreferably incorporates less than about 1 wt % of such additional,conventional adsorbents, greatly reducing costs and complexity.

In order to enhance such excellent dispersion of clay particles withinthe polymer matrix, it can be advantageous to incorporate between about0.2 wt % and about 0.6 wt % of an emulsifier, preferably analkyl-trimethyl-ammonium surfactant, such as cetyl-trimethyl-ammoniumbromide (CTAB), with the clay and polymer upon melt mixing. Thoseskilled in the art will recognize that many other such emulsifiers canbe used, so long as they are stable at the melt processing temperaturesused to melt mix the adsorbent clay particles and polymers.

One manner of making the new adsorbent composition is to mill clay to asize below about 100 mesh and wash the clay particles with distilledwater several times to remove impurities. The clay is then dried in avacuum oven overnight. The dried clay is composited with polymerparticles, and optionally an emulsifier, dry blended and then fed to anextruder at a temperature above the melting or softening point of theparticular polymer in use, such as above about 180° C., even above 190°C., or even above 250° C. The extrudate is collected in a water bath,dried and subsequently ground to granules of different sieve sizes,ranging for example from about 0.5 mm to about 3 mm. When an emulsifieris used, it can be incorporated at concentration levels of about0.3g/100 grams of the other components.

The granulated adsorbent composition can be packed into a separationcolumn or other such suitable vessel, either in the dry state or bymixing with water and allowing the packing to settle into a packed bedtoward the bottom of the vessel. As illustrated in the FIGURE, heavymetal contaminated water which is stored in a vat 101 is withdrawnthrough pump 102 and fed under pressure to column 103 containing theadsorbent composition particles. The amount of pressure can be variedbased upon the length and width of the column, the adsorbent compositionparticle size and the overall requirements of the removal process. Theapplied pressure should not be so high as to cause the adsorbentcomposition packing to collapse or to form channels within the packing.Treated water is withdrawn at output valve 104, while some treated wateris recycled through recycle line 105 back into vat 101. Recyclingtreated water increases the contact time of lead ions with clayparticles. In most cases it was found that samples withdrawn afterhalf-an hour achieved good percentage of lead removal.

Thus, in another embodiment the invention is directed to a method forremoving heavy metals from contaminated water, comprising passing astream of water having a concentration of one or more heavy metalsthrough an adsorbent composition as disclosed above, comprising granulesof a mixture of from about 1 wt % to about 15 wt %, even between about 3wt % and about 10 wt % clay, or even between about 4 wt % and 7 wt %,such as about 5 wt % clay and a thermoplastic polymer matrix, preferablyhaving less than about 1 wt % of additional adsorbents; and collectingwater having a reduced concentration of said heavy metal(s) downstreamof said adsorbent composition.

The thermoplastic polymer can be any theremoplastic polymer which can bemelt processed, such as by a melt extruder, and is advantageouslypolyethylene terephthalate (PET) or polystyrene (PS), which have beendemonstrated to have a good affinity for dispersion of clay particles.

The removal method can be enhanced by passing said stream of heavy-metalcontaminated water through said adsorbent composition packing underapplied pressure, which can be adjusted based upon the length and widthof the column, the adsorbent composition particle size and the overallrequirements of the removal process. The applied pressure should not beso high as to cause the adsorbent composition packing to collapse or toform channels within the packing. In a preferred mode, the solutionbeing treated is maintained at a pH>3, is treated for at least about 30minutes at ambient temperature or above. Higher treatment temperatureswill result in better adsorption.

The removal method preferably utilizes clay particles which have anaturally-occurring ion-exchange capacity, such as for example an Illiteclay, and can be preferably practiced in a continuous mode, such that acontinuous stream of said heavy-metal contaminated water is passedthrough said adsorbent composition packing under applied pressure.

The removal method is effective in removing heavy metals including lead,chromium, copper, zinc, cadmium and combinations thereof. We have foundthat when the heavy metal is lead the concentration of lead in saidwater is reduced by more than about 30%, even as much as about 56%.

Example 1

Illite clay in the form of soft rocks was received and milled to aparticle size below 100 mesh, washed with distilled water several timesand dried in a vacuum oven over night. The dried clay particles werethen dry blended with PS particles and CTAB emulsifier at concentrationsof about 5 wt % clay particles and about 0.3 wt % CTAB, relative to theweight of the entire composition. The mixture was then fed to a Lab MinExtruder (LME) manufactured by Dynisco Company at a temperature of about190° C. and extruded into a water bath to solidify the extrudate. Theextrudate was dried, cut into pellets and then ground into particlessized between about 0.5 mm to 3 mm. 2 to 4 grams of adsorbent particlesso-formed were packed in the column as shown in FIG1, then 300-700 mLwater contaminated with 100 ppm Pb was circulated through the adsorbentbed at ambient temperature. The samples were drawn every thirty minutesusing the valve 104 in FIG. 1. The lead content of the water was testedby atomic absorption spectroscopy and determined to be less than 70 ppm,the removal efficiency being greater than 30% [˜38%-55%].

Example 2

The dried clay particles of Example 1 were dry blended with PETparticles and CTAB emulsifier at concentrations of about 5 wt % clayparticles and about 0.3 wt % CTAB, relative to the weight of the entirecomposition. The mixture was then fed to a Lab Min Extruder (LME)manufactured by Dynisco Company at a temperature of about 285° C. andextruded into a water bath to solidify the extrudate. The extrudate wasdried, cut into pellets and then ground into particles sized betweenabout 0.5 mm to 3 mm. 2-4 grams of adsorbent particles so-formed werecombined with 300-700 mL water contaminated with 100 ppm Pb, and stirredin a batch process for about 30 minutes at a ambient temperature. Afterstirring was complete, the lead content of the water was tested anddetermined to be less than 80 ppm, the removal efficiency being greaterthan about 20%.

The foregoing examples have been provided for the purpose of explanationand should not be construed as limiting the present invention. While thepresent invention has been described with reference to an exemplaryembodiment, changes may be made within the purview of the appendedclaims, without departing from the scope and spirit of the presentinvention in its aspects. Also, although the present invention has beendescribed herein with reference to particular materials and embodiments,the present invention is not intended to be limited to the particularsdisclosed herein; rather, the present invention extends to allfunctionally equivalent structures, methods and uses, such as are withinthe scope of the appended claims.

What is claimed:
 1. An adsorbent composition for metal ions, comprisinga granulated mixture of from about 1 wt % to about 15 wt % clay and athermoplastic polymer matrix and between about 0.2 wt % and about 0.6 wt% of an emulsifier.
 2. The adsorbent composition of claim 1, having lessthan about 1 wt % of additional adsorbents.
 3. The adsorbent compositionof claim 1, wherein the thermoplastic polymer has a melting or softeningpoint above about 180° C.
 4. The adsorbent composition of claim 3,wherein the thermoplastic polymer is polyethylene terephthalate orpolystyrene.
 5. The adsorbent composition of claim 1, wherein the clayis present at a concentration between about 3 wt % and 10 wt %.
 6. Theadsorbent composition of claim 5, wherein the clay is present at aconcentration between about 4 wt % and 7 wt %.
 7. The adsorbentcomposition of claim 1, wherein the clay has a naturally-occurringion-exchange capacity.
 8. The adsorbent composition of claim 1, whereinthe clay is an Illite clay.
 9. The adsorbent composition of claim 1,wherein the emulsifier is cetyl-trimethyl-ammonium bromide.
 10. Theadsorbent composition of claim 1, wherein the granulated mixturecomprises granules in the size range from about 0.5 mm to about 3 mm.